This invention relates to apparatus for generating a velocity command signal and, more particularly, to an apparatus for generating a velocity command signal representing a predetermined optimum velocity of movement of a selected one of a plurality of movable elements from its actual position to a desired stopping position.
Servo control systems have been frequently employed to move a movable element from its actual position to a desired stopping position. One type of servo control system is the so-called "linear-mode" servo which is normally characterized by the comparison of a first signal indicative of the desired position for the movable element and a second signal indicative of the actual position of the movable element with appropriate damping. A position error signal representative of such comparison is then generated and is used to drive a servo motor and thus the movable element in the proper direction. Another type of servo control system is the so-called "non-linear-mode" servo which is normally characterized by the comparison of a first signal indicative of a desired velocity at which the movable element is to be moved for any particular distance remaining to be traveled to a desired stopping position and a second signal indicative of the actual instantaneous velocity of movement of the movable element. A velocity error signal representative of such comparison is then generated and is used to drive the servo motor in the appropriate direction.
Certain machines having movable elements require extremely accurate positioning of such movable elements. An example of such machine is a high speed serial printer of the type employing a rotatable print wheel mounted to a linearly movable carriage. Both the rotational position of the print wheel and the linear position of the carriage must be very accurately determined and attained. With these machines, it has pg,3 been found desirable to use yet another type of servo control system, commonly referred to as a "dual-mode" servo. In this servo, the positioning of the movable element is controlled first in a non-linear mode wherein an actual velocity signal is compared with a desired velocity signal (hereinafter referred to as a velocity command signal) and then a velocity error signal is generated to drive the servo motor. As the particular movable element reaches a predetermined position relative to the desired stopping position, the servo switches to a linear mode of operation where a third signal indicative of the actual incremental position of the movable element is compared with the second signal indicative of the actual velocity of such element and the resultant error signal drives the servo motor. The actual velocity signal performs strictly a damping function in the linear mode, as is conventional.
It will thus be apparent that both the non-linear-mode and dual-mode servos alluded to above require the generation of a velocity command signal indicative of a desired velocity of movement of the controlled movable element for a particular distance remaining to be traveled. The ideal velocity command signal would be that which is derived from a real time solution of the differential equation of motion for the particular movable element involved. This differential equation may be expressed generally as follows: ##EQU1## where K.sub.1, K.sub.2 and K.sub.3 are constants together defining the unique motion characteristics of the particular movable element in terms of its mass and other parameters, and x is the distance remaining to be traveled.
As is well known, an approximation of the ideal velocity command signal can be obtained through a real time solution of the equation: EQU y= K.sqroot.x,
where K is a single constant defining the unique motion characteristics of the particular movable element, x is the distance remaining to be traveled and y is the velocity command signal.
U.S. Pat. No. 3,639,754 discloses a system for computing a so-called stopping pattern signal for a vehicle. As described in the patent, the stopping pattern signal is generated electronically in accordance with the term K.sqroot.x, where the value .sqroot. x is generated as the digital output of a .sqroot.x counter and is then converted into analog form by a D/A converter. This analog signal is then attenuated in accordance with a resistor circuit in order to effectively multiply the analog .sqroot. x signal by a constant K.
Examples of other systems which include apparatus for generating a velocity command signal satisfying a desired velocity-distance profile are disclosed in U.S. Pat. Nos. 3,737,751 and 3,582,629.
Among the machines requiring the control of a plurality of movable elements is the one disclosed in U.S. Pat. No. 3,821,629. Movement of each of the movable elements of that machine is controlled by a separate linear-mode (position) servo system. A plurality of position command signals are calculated for the required movements of the plurality of movable elements and are supplied on a time-shared basis to the respective plurality of servo control systems.
As alluded to above, another machine having a plurality of controllably movable elements is a high speed serial printer of the type including a rotatable print wheel mounted to a linearly movable carriage. Prior art high speed printers of this type have used two separate velocity command signal generating apparati to generate the two separate velocity command signals required. Although this arrangement has proven effective from an operational standpoint, it is clear that the use of duplicative circuits adds to the cost and complexity of the system.
In addition to the above, many prior art velocity command signal generating apparati, particularly those used in high speed serial printers, have used non-linear feedbacks elements in at least one stage thereof. These elements are typically steered by diode networks. When the inaccuracies of the non-linear elements and diode networks are added to the uncertainties of the movable elements, it is necessary to provide a means of adjusting the system for reliable operation.
It would be desirable, therefore, to provide a single velocity command signal generating apparatus capable of generating a plurality of velocity command signals for each of a plurality of servo control systems in order that the movements of a respective plurality of movable elements may be controlled. It would further be desirable if such single velocity command signal generating apparatus were devoid of non-linear feedback elements and diode steering networks.